1. Field of the Invention
The present invention relates in general to a shift control apparatus for a vehicle step-variable automatic transmission having a plurality of selectable groups of forward-drive positions having respective different sets of speed ratios, and more particularly to techniques associated with the shift control apparatus, for improving the drivability of a vehicle by suitably selecting one of the groups of forward-drive positions of the automatic transmission upon switching of its shifting mode from an automatic shifting mode to a manual shifting mode.
2. Discussion of Related Art
A known shift control apparatus for a vehicle step-variable automatic transmission has a so-called normal shifting pattern suitable in a normal drive mode of a motor vehicle, wherein the automatic transmission is shifted to one of a plurality of selectable operating positions, according to a predetermined relationship between a running speed of the vehicle and an opening angle of a throttle valve of an engine of the vehicle, which relationship is determined for a normal or ordinary running condition of the vehicle. Where the shift control apparatus has only the normal shifting pattern, the shift control apparatus may not intricately deal with different specific running conditions of the vehicle. In view of this drawback, JP-59-50261A proposes a shift control apparatus having shifting patterns other than the normal shifting pattern, such as a so-called power-drive shifting pattern selected in a power-drive mode for higher drivability of the vehicle than in the normal drive mode, and a so-called economy-drive shifting pattern selected in an economy-drive mode for higher fuel economy of the vehicle than in the normal drive mode. According to this shift control apparatus, an appropriate one of the different shifting patterns is selected to shift the automatic transmission, on the basis of a change in the opening angle of the throttle valve, so that the automatic transmission can be intricately controlled according to the shifting pattern that suits the specific running condition of the vehicle. However, the different shifting patterns merely have different shifting points (different combinations of running speed and throttle valve opening angle) at which shift-up and shift-down actions of the automatic transmission take place, so that the freedom or flexibility of shifting of the automatic transmission is lower than in a manual transmission which is shifted as desired by the operator of the vehicle.
In view of the drawback of the shift control apparatus disclosed in JP-59-50261A, JP-5-65953A discloses a shift control apparatus including a manually operable shifting device (shift-lever device) having not only an auto-shift position (D) for an automatic shifting mode in which the automatic transmission is automatically shifted to a selected one of a plurality of operating positions, but also a manual-shift position for a manual shifting mode in which the automatic transmission is manually shifted by the vehicle operator. In the manual-shift position, the operator can select one of different shifting patterns corresponding to respective groups of operating positions that are different from a group of operating positions available in the auto-shift position (D). In the manual shifting mode selected when the manually operable shifting device is placed in the manual-shift position, the automatic transmission can be manually shifted by the vehicle operator to a selected one of the operating positions of the selected shifting pattern. The operating position selected in the manual shifting mode may have a higher speed ratio than the corresponding operating position in the automatic shifting mode, for a higher value of acceleration or drivability of the vehicle as desired by the vehicle operator.
However, the shift control apparatus disclosed in JP-5-65953A suffers from an undesirable change in the speed ratio of the automatic transmission upon switching of its shifting mode from the automatic shifting mode to the manual shifting mode. Namely, since the speed ratios of the operating positions selectable in the manual shifting mode are different from those selectable in the automatic shifting mode, the speed ratio of the operating position selected upon switching to the manual shifting mode is different from that of the operating position selected last in the automatic shifting mode. Further, the above-identified publication discloses only one group of operating positions having a single set of speed ratios available in the automatic shifting mode, and does not disclose selection of one of a plurality of groups of operating positions having respective different sets of speed ratios, in the manual shifting mode, where a plurality of groups of operating positions are also available in the automatic shifting mode. That is, where the two or more groups of operating positions having respective different sets of speed ratios are available in the automatic shift mode, the set of speed ratios of the group of operating positions selected in the manual shifting mode upon switching of the shift mode from the automatic shifting mode to the manual shifting mode may be different from the set of speed ratios of the group of operating positions selected last in the automatic shifting mode. In this case, the switching of the shifting mode causes an undesirable change in the speed ratio of the automatic transmission.